Vinylidene chloride interpolymer as a coating for regenerated cellulose film



Patented Feb. 13, 1951 2,541,167

VINYLIDENE CHLORIDE INTERPOLYMER AS 1 A COATING FOR REGENERATED CELLU-LOSE FILM Gilbert Pitzl, Eggertsville, N. Y., assignor. to E. L

du Pont de Nemours & Company, Wilmington, Del., a corporation ofDelaware No Drawing. Application June 25, 1948,

' Serial No. 35,295

4 Claims.

, odorless, transparent, heat sealable and inherentlymoisture-resistant, thin coatings oi! vinylidene chloride/acrylonitrilecopolymers particularly recommend themselves for this purpose. Coatingsof vinylidene chloride/acrylonitrile copolymers comprising at least 80%vinylidene chloride have excellent moistureproofness but their adhesionto the base film under high moisture conditions such as are encounteredwhen the coated film is wrapped around products containing considerablewater (cheese, fish, fresh vegetables, etc.), is very poor. In order toimprove the adhesion, it is currently necessary to first apply ananchoring sub-coating and then the vinylidene chloride/acrylonitrilecoating, which represents an extra processing step, and hence acommercial handicap. I

An object of this invention thereforei's' to improve the adherence oranchorage to the base film of transparent, moistureproof heat-scalablecoatings oi vinylidene chloride copolymer.

Another object is to provide a regenerated cellulose film having indirect contact therewith a well adhered moistureproof, heat-scalable,transparent coating of vinylidene chloride copolymer.

Still another object is to provide moistureproof, heat-scalabletransparent coatings of vinylidene chloride copolymer which copolymersstrongly adhere directly to base materials, such as regeneratedcellulose film, even when the coated base material is maintained in anatmosphere of high moisture content, or in direct contact with water forconsiderable periods of time. These and other objects will more clearlyappear hereinafter.

After extensive research I have found that the three-component,hydroxyl-containing copolymers formed by interpolymerizing a criticallyproportioned mixture 01 vinylidene chloride, acrylonitrile, and2-hydroxyl ethylacrylate mono- (c1. zoo-17.4)

mers, or by copolymerizing a monomer mixture of vinylidene chloride,acrylonitrile, and vinyl acetate and thereafter hydrolyzing orsaponifying the acetate groups, constitute excellent mois tureproofing,heat-sealable transparent coatings for regenerated cellulose, and areoutstanding in their ability to bond directly and strongly toregenerated cellulose film, which bond i highly resistant to thedeteriorating influence of high moistur or water.

Accordingly the above objects are achieved by coating a base film orsheet of regenerated cellulose, by any convenient coating technique.with a coating composition comprising essentially a copolymer ofvinylidene chloride, acrylonitrile and z-hydroxy ethylacrylate, orhydrolyzed vinyl acetate.

In order to secure self-anchoring copolmers (i. e. copolymers capable ofstrongly adhering directly to regenerated cellulose) having therequisite degree of moistureproofness, heat-sealability, flexibility,etc., it is essential that the initial relative proportions of monomersin the reaction mixtur be within thefollowing ranges: The 2-hydroxyethylacrylate, or vinyl, acetate content of the reaction mixture may bevaried from 1% to 15%; below 1% the anchorage of the resultingcopolymer, under high moisture, conditions, is unsatisfactory, andcopolymers resulting from a reaction mixture containing in excess of 15%2-hydroxy ethylacrylate or vinyl acetate are not moistureproof.Acrylonitrile should constitute from 5 to 15% of the monomer charge.Less than 5% of acrylonitrile results in insolubility, and more than 15%imparts; interior moistureproofness. The vinylidene chloride of courseconstitutes the remaining to 94% of the monomer charge.

The monomers may be copolymerized by any known method to form the threecomponent interpolymersof this invention. For example, the

and/or dispersing agents. Alternatively, the copolymers of thisinvention may be prepared by polymerization of the monomericcomponentsln bulk without added diluent, or the monomers may be reactedin appropriateorganic solvent reaction media. The totalcatalyst-activator concentration should generally be kept within a rangeof about 0.01 to about 2.0% by weight of the monomercharge, andpreferably within a range of concentration of 0.1% to 1.0% Improvedsolubility and viscosity values are obtained by conducting thepolymerization in the presence of mercaptans such as ethyl mercaptan,lauryl mercaptan,' .tertiar'y dodecyl mercaptang etc., which areeffective in-reducing cross-linking in the copolymer. In general, themercaptans should be used in concentrations of 0.1% to 5.0% by weight,based on the weightof polymerizable monomers present in the charge.

When the copolymer is prepared from vinyl acetate (together withvinylidene chloride and acrylonitrile) it is necessary'to subject theco-- polymer to a hydrolysis reaction to convert the acetate groups tothe desired hydroxyl groups. While any known method for convertingpolyvinyl acetate to polyvinyl alcohol may be employed, a convenientprocedure is to saponify the interpolymer, using a mineral acid, such assuliuric acid as the hydrolyzing agent (U. 8.

Application'of these novel copolymers to the base film may be madeeither from an organic solvent solution or from an aqueous dispersion ofthe copolymers, and'by any convenient coat- 4 EXAMPLE 11 A chargeconsisting of:

V Parts Water 2300 Ammonium persulfate 9.6

Sulfated methyl oleate 45 Acrylonitrile 60 2-hydroxyli ethylacrylate-..60

' vinylidene j chloride 480 is heated to 33 C. under reflux and stirringand 0.64 parts of meta sodium bisulfite in parts of water is added. Areaction temperature of 32-35 C. is maintained fo 56 hours, after whichtime the emulsion is drawn off, coagulated with aluminum sulfate and thepolymer purified. These conditions produce an approximate 80/10/10vinylidene chloride/ acrylonitrile/ 2 hydroxyl ethylacrylate copolymer,which copolymer is soluble in a 50/50 methyl ethyl ketone/toluenemixture.

The results of evaluation of these two copolymers as coatings onregenerated cellulose film 1 mil thick appear in Table I.

Table I Polymer Solubility Heat Seals-35% an Anchorage in H10- 25 C. (3)

Initial (l) Peel (2) Hydrolyzed product of Exam eI. Polymer oi ExampleII Control-90110 vinylidene chloride/acryionitrlle.

801. in Lil-dioxane So]. in mixture of methyl ethyl ketone and toluene.Sol. in methyl ethyl ketone.

OK-l day: stripped-2 days. Fair.

Very Poor.

ing expedient such as dipping, roll-coating, brushing, spraying, etc.,followed, where necessary, by a solvent-removal or drying step.

The following examples of preferred embodiments further illustrate theprinciples and practice of this invention. Parts and percentages are byweight unless otherwise indicated.

is heated to 33 C. under reflux and stirring and 4.8 parts of metasodium bisulfate in 25 parts of water is added. A reaction temperatureof 32-35 C. is maintained for '16 hours after which time the emulsion isdrawn ofl, coagulated with aluminum sulfate and the polymer purified.These conditions produce an approximate 82/8/10 vinylidenechloride/acrylonitrile/vinyl acetate copolymer in yields of 80% whichinterpolymer is soluble in dioxane.

50 parts of the above dried polymer is dissolved in 400 parts of dioxaneat 50 C. Then, under reflux and stirring, 160 parts of methanol adjustedto a pH of 2 with H2804 is slowly added. After addition is completed,the mixture is heated to 70 C. and allowed to react under reflux for 8hours. The polymer is coagulated with water and purified. Thesaponification of vinyl ester copolymers using a mineral acid, such assulfuric acid. as a hydrolyzing agent is old in the art. (See U. 8.2,399,653 or French 724,910).

The above data show significant improvement in adhesion for thehydroxyl-containing interpolymers as compared to the control.

(1) The definition of and tests for determining heat seal bond strengthare set forth in U. S. 2,236,546 (Mitchell).

(2) Whereas the initial heat seal value is the force in grams requiredto just rupture the seal, the peel heat seal value is the force requiredto separate the seal after it has been ruptured. It of course, takesmore strength to rupture the heat seal initially than it does toseparate the seal after it has once been ruptured.

(3) Anchorage-refers to the adhesion of the coating to the base sheetwhen in direct contact with water. It is more fully defined in U. S.2,301,959 (Lanning).

While the invention contemplates the use of the novel copolymers hereinas the essential filmformer constituent of the coating or coatingcomposition, it is to be understood that in many instances it will bedesirable to incorporate in the coating or coating composition otherefiect materials such as slip agents, pigments, dyes, plas ticizers,delustrants etc., and the addition of such materials is thereforeunderstood to be within the purview of this invention.

The novel vinylidene chloride coatings of this invention, as shownhereinabove, strongly adhere directly to regenerated cellulose base filmunder substantially all conditions encountered in the use of transparentwrapping tissues, and thus enable the obtainment of vinylidene chloridecopolymer-coated regenerated cellulose film without the necessity ofapplying to the base film a costly anchoring subcoat. Moreover coatedfilms of this invention have superior heat-seal characteristi'cs whichof course enhance their value in the art of transparent wrapping tissueAs many widely diflerent embodiments can be made without departing fromthe spirit and scope or my invention, it is understood that saidinvention is not limited in any way except as delined in the appendedclaims.

I claim: Q

1. A moistureprooi, heat-scalable wrapping tissue comprising regeneratedcellulose film having a self-anchored coating comprising essentially acopolymer obtained by polymerizing a mixture of 1-15% of vinyl acetate,5-15% of acrylonitrile, and 70-94% of vinylidene chloride, theproportions being selected to total 100%, and thereafter hydrolyzing toconvert the acetate groups of the resulting copolymer to hydroxy E HDB-r 2. The product 01' claim 1 wherein the polymerizable monomer mixtureconsists of 1.0% vinyl acetate, 10% acrylonitrile, and 80% vinylidenechloride. 1 n

3. A copolymer obtained by polymerizing a mixture of 1-15% ofvinylacetate, 545% of acrylonitrile, and 70-94% of vinylidene chloride, andthereafter hydrolyzing to convert the acetate groups or the resultingcopolymer to hydroxy groups.

4. A copolymer obtained by hydrolyzing a copolymer consisting of 82%vinylidene chloride, 8% acrylonitrile and 10% vinyl acetate.

GILBERT PITZU.

No references cited.

1. A MOISTUREPROOF, HEAT-SEALABLE WRAPPING TISSUE COMPRISING REGENERATEDCELLULOSE FILM HAVING A SELF-ANCHORED COATING COMPRISING ESSENTIALLY ACOPOLYMER OBTAINED BY POLYMERIZING A MIXTURE OF 1-15% OF VINYL ACETATE,5-15% OF ACRYLONITRILE, AND 70-94* OF VINYLIDENE CHLORIDE, THEPROPORTION BEING SELECTED TO TOTAL 100%, AND THEREAFTER HYDROLYZING TOCONVERT THE ACETATE GROUPS OF THE RESULTING COPOLYMER TO HYDROXY GROUPS.